Trends in the design of multi-chip modules and related electronic assemblies continue to emphasize the elimination of individual packages from semiconductor devices to allow more devices in less circuit board space. This results in improved electrical performance but causes an increase in the power density of the multi-chip module. Increased power density translates to higher module operating temperatures requiring more elaborate thermal design and thermal management schemes to prevent the device from overheating. Thermal management deals with a variety of means to remove heat from the proximity of active semiconductor devices. Suitable cooling means include heat conductive metal structures, such as heat sinks or heat spreaders, and fluid materials such as thermal greases and liquid coolants. Efficient thermal management requires contact or at least close proximity between cooling means and electronic assembly. The prior art includes multiple approaches towards the goal of efficient thermal management. U.S. Pat. No. 4,409,641 discloses a printed circuit, connecting multiple semiconductor devices, inside the cavity of an open-topped, electrically insulating housing. A metal cover completes the enclosure which fully surrounds the printed circuit. Heat removal from the enclosure requires a heat-conductive connection from metal surfaces inside the housing cavity to the cover which then loses heat by radiation. U.S. Pat. No. 4,811,165 also uses a metal plate to conduct heat from semiconductor devices attached to a flexible printed circuit material. The circuit assembly is fully enclosed by covers attached to either side of the heat conducting metal plate. U.S. Pat. Nos. 5,243,131 and 5,353,194 provide further evidence of the use of heat conducting metal structures for heat removal
Cooling efficiency, using heat conducting plates, is hindered by lack of direct contact between the heat conducting, cooling element and the bodies of the heat generating semiconductor devices. To overcome this deficiency, a conductive paste or grease may bridge the space, between the devices and heat conducting surface, to facilitate the desired heat transfer. U.S. Pat. Nos. 5,109,317; 5,208,733; 5,289,337 and 5,777,847 describe structures that use a thermal grease or paste to enhance cooling.
Another method of removing heat from electronic assemblies, such as multi-chip modules, relies upon the circulation of cooling liquids close to the active semiconductor devices. Enclosures designed for this type of cooling must provide liquid circulation without leakage, thereby adding to the complexity of the design. Fully enclosed heat dissipating housings, using liquid cooling, include those described in U.S. Pat. Nos. 4,492,878 and 5,109,317.
With the exception of U.S. Pat. No. 5,777,847, the prior art, previously described, uses enclosures that fully surround the multi-chip modules or circuit components. In contrast, U.S. Pat. No. 5,777,847 provides a protective cover plate for a multi-chip module with semiconductor devices attached to a printed circuit board. Attachment of the cover plate to the circuit board uses at least one pillar held in contact with the board by an adhesive or a retaining pocket, sized to the cross-section of the pillar, or a screw connection of the base of the pillar into the circuit board. Each attachment means is subject to separation from the circuit board due, in one case, to adhesive failure or, in other cases, to loosening of mechanically held pillars. The pillar produces a space between the device-carrying surface of the printed circuit board and the underside of the cover plate so that the space extends to the perimeter of the circuit board. The cover plate may be metallic and include several plates in spaced parallel arrangement to increase cooling efficiency. With this design, heat may escape by air convection from the sides of the multi-chip module, as well as thermal conduction through the heat conducting cover plate structure. Despite its open structure the multi-chip module requires a thermal grease between the semiconductor devices and the cover plate to improve the rate of heat removal.
The current invention addresses disadvantages of previous thermal management schemes by providing a multi-chip module protecting package that includes at least one cover plate latched to the circuit board and having a spring biased heat spreader interposed between and in contact with the semiconductor devices and the inside of the cover.
In contrast to covers disclosed in U.S. Pat. No. 5,777,847, the cover plates described herein have little tendency to detach from the circuit board. This advantage of the current invention arises from the use of special latches having flexible posts inserted into mounting holes drilled through the circuit board. While passing through the mounting holes, the arrowhead-shaped tips of the flexible posts experience lateral compression until exiting on the opposite side of the board, when a stepped portion of the tips snaps against the exit surface to secure the latch in the mounting hole. In the absence of other design considerations, the cover cannot be removed after latch installation. The latch design allows space for two latches, in suitable orientation, to simultaneously occupy the same circuit board mounting hole. This orientation requires insertion of each one of a pair of latches from opposite sides of the circuit board, thus simplifying attachment of covers to either side of the board. Use of common mounting holes for attaching multiple covers further contributes to the desirable objective of free board space to add more semiconductor devices or reduce the area occupied, and hence the size, of a multi-chip module.